Reduction gear transmissions that are compact yet are capable of achieving large reduction ratio are being developed. Japanese Patent Application Publication No. 62-004586 sets forth a reduction gear transmission that comprises a motor, an input shaft, a crankshaft, an external gear, and an internal gear. A motor gear is fixed to an output shaft of the motor. The input shaft extends parallel to the output shaft of the motor, and an input gear and an output gear are formed thereof. The input gear meshes with the motor gear. The crankshaft comprises a gear that meshes with the output gear, and an eccentric rotation member that rotates eccentrically around an axis line of the crankshaft when the aforementioned gear rotates. The external gear comprises a through hole formed at a position offset from a center of the external gear, and outer teeth formed on its outer circumference face. The eccentric rotation member of the crankshaft is inserted into the through hole of the external gear. When the crankshaft rotates, the external gear revolves orbitally around an axis line of the input shaft. The internal gear comprises inner teeth formed on its inner circumference face, and meshes with the outer teeth of the external gear while surrounding the external gear in a state allowing the orbital revolution of the external gear. The number of teeth of the external gear differs from the number of teeth of the internal gear.
In this type of reduction gear transmission, it is possible to freely adjust the reduction ratio by adjusting the number of teeth of the external gear and the number of teeth of the internal gear. As a result, a configuration has been adopted in which adjustment so as to obtain the desired reduction ratio is performed by adjusting the number of teeth of the external gear and the number of teeth of the internal gear, whereas the motor gear fixed to the output shaft of the motor meshes with the input gear fixed to the input shaft.
Similar reduction gear transmissions are also known; whereas these reduction gear transmissions comprise a motor, an input shaft, an ellipsoidal rotation member, an external gear, and an internal gear.
The input shaft extends parallel to an output shaft of the motor. The ellipsoidal rotation member is fixed to the input shaft in a manner that forms an ellipsoid shape when viewed along an axis line of the input shaft. The external gear is capable of changing its shape elastically, and engages at its inner side with an outer face of the ellipsoidal rotation member. The internal gear surrounds the external gear while meshing with the external gear at a position along the major axis of the ellipsoidal rotation member, while allowing the ellipsoidal rotation member to rotate around the axis line of the input shaft. The number of teeth of the external gear differs from the number of teeth of the internal gear.
In this type of reduction gear transmission, the reduction ratio can likewise be adjusted freely by adjusting the number of teeth of the external gear and the number of teeth of the internal gear. As a result, a configuration has been adopted in which adjustment so as to obtain the desired reduction ratio is performed by adjusting the number of teeth of the external gear and the number of teeth of the internal gear, whereas the output shaft of the motor and the input shaft are joined directly together.
In the conventional reduction gear transmission, a reduction gear transmission having the desired reduction ratio is realized by utilizing the fact that the desired reduction ratio can be obtained by adjusting the number of teeth of the external gear and the number of teeth of the internal gear. As a result, additional reduction gear mechanisms besides the external gear and the internal gear are not required, and the configuration in which the output shaft of the motor and the input shaft are joined directly together is adopted. Alternatively, the configuration in which the motor gear fixed to the output shaft of the motor meshes with the input gear of the input shaft is adopted.